Heat-resistant cast steel, and preparation method and use thereof

ABSTRACT

The present invention provides a heat-resistant cast steel, and a preparation method and use thereof. Based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel includes the following elements in mass percentage: 0.08 wt %-0.18 wt % of C, 0.10 wt %-0.40 wt % of Si, 0.30 wt %-0.70 wt % of Mn, 9.80 wt %-10.70 wt % of Cr, 3.00 wt %-3.50 wt % of Co, 1.60 wt %-2.00 wt % of W, 0.45 wt %-0.85 wt % of Mo, 0.10 wt %-0.30 wt % of V, 0.02 wt %-0.08 wt % of Nb, 0.010 wt %-0.035 wt % of N, 0.001 wt %-0.010 wt % of B, &lt;0.20 wt % of Ni and 79 wt %-85.5 wt % of Fe. The heat-resistant cast steel can satisfy the use requirements of turbine parts with a working temperature of 635° C. and below 635° C.

TECHNICAL FIELD

The present invention relates to the field of metal materials, in particular to a heat-resistant cast steel, and a preparation method and use thereof.

BACKGROUND ART

A steam turbine in turbomachinery, also known as a steam turbine engine, is a rotary steam power unit. At the steam turbine, high-temperature high-pressure steam passes through a fixed nozzle to become an accelerated steam flow which is then directed onto blades, so that the rotor with a row of blades rotates and does external work. The steam turbine is the main equipment of a modern thermal power plant.

Improving the steam temperature parameters of a coal-fired unit in the thermal power plant may increase the unit efficiency, reduce the consumption of fossil fuels and achieve energy conservation and emission reduction. The operating temperature of the steam turbine is limited by the maximum operating temperatures of the materials of key components (cylinders, valves, rotors, blades, etc.).

High-temperature casting materials used for steam turbine cylinders and valve casings have been developed from Cr-Mo steel to various 9%-12% Cr ferritic steels. Among the existing high-temperature casting materials, ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB and the like are currently available options. The maximum working temperature of steel grade ZG12Cr10Mo1W1VNbN cannot exceed 610° C., and the maximum working temperature of steel grade ZG13Cr9Mo2Co1NiVNbNB cannot exceed 625° C. As a result, there is currently no heat-resistant cast steel material for steam turbine castings that can satisfy the working temperature of 635° C.

SUMMARY OF THE INVENTION

In view of the above defects in the prior art, an objective of the present invention is to provide a heat-resistant cast steel, and a preparation method and use thereof in order to solve the problems in the prior art.

The above objective and other related objectives of the present invention are achieved by the technical solutions as follows.

The present invention provides a heat-resistant cast steel. Based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel includes the following elements in mass percentage:

0.08 wt %-0.18 wt % of C, 0.10 wt %-0.40 wt % of Si, 0.30 wt %-0.70 wt % of Mn, 9.80 wt %-10.70 wt % of Cr, 3.00 wt %-3.50 wt % of Co, 1.60 wt %-2.00 wt % of W, 0.45 wt %-0.85 wt % of Mo, 0.10 wt %-0.30 wt % of V, 0.02 wt %-0.08 wt % of Nb, 0.010 wt %-0.035 wt % of N, 0.001 wt %-0.010 wt % of B, <0.20 wt % of Ni and 79 wt %-85.5 wt % of Fe.

According to the technical solution described above in the present application, the mass percentage of Fe is 80.5 wt %-84.7 wt %.

According to the technical solution described above in the present application, the mass percentage of Fe is 81 wt %-83.8 wt %.

According to the technical solution of the heat-resistant cast steel described above in the present application, the heat-resistant cast steel further contains impurities, including one or more of Al, P, S, Cu, Ti and Sn.

According to the technical solution of the heat-resistant cast steel described above in the present application, based on the total mass of the heat-resistant cast steel, the corresponding mass percentages of Al, P, S, Cu, Ti and Sn are: ≤0.030 wt % of Al, ≤0.030 wt % of P, ≤0.020 wt % of S, ≤0.25 wt % of Cu, ≤0.030 wt % of Ti and ≤0.030 wt % of Sn, more preferably, ≤0.020 wt % of Al, ≤0.020 wt % of P, ≤0.015 wt % of S, ≤0.15 wt % of Cu, ≤0.020 wt % of Ti and ≤0.020 wt % of Sn.

According to the technical solution of the heat-resistant cast steel described above in the present application, based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel includes the following elements in mass percentage: 0.10 wt %-0.16 wt % of C, 0.20 wt %-0.30 wt % of Si, 0.40 wt %-0.60 wt % of Mn, 10.00 wt %-10.50 wt % of Cr, 3.10 wt %-3.40 wt % of Co, 1.65 wt %-1.90 wt % of W, 0.55 wt %-0.75 wt % of Mo, 0.15 wt %-0.25 wt % of V, 0.03 wt %-0.07 wt % of Nb, 0.015 wt %-0.030 wt % of N, 0.002 wt %-0.008 wt % of B, ≤0.10 wt % of Ni and the balance of Fe and impurity elements.

The present invention further discloses use of the above-described heat-resistant cast steel in turbomachinery, especially use as a casting material in the field of steam turbines.

The reasons for limiting the mass percentages of the elements in the heat-resistant cast steel provided by the present invention are as follows:

Carbon (C): C ensures hardenability. In the tempering process, after C is combined with Mo, W and other elements, M23C6 carbides are formed at grain boundaries and martensite boundaries, and after C is combined with Nb, V and other elements, MX carbonitrides are formed in the martensite. After precipitation strengthening of the above M₂₃C₆ carbides and MX carbonitrides, the high-temperature strength can be increased. In addition to ensuring strength and toughness, C is also an indispensable element to inhibit the formation of harmful phases δ-ferrite and BN. In order to make the heat-resistant cast steel of the present invention have the required strength and toughness, the C content should be 0.08% or above. However, an excessive addition will reduce the toughness of the steel, and an excessive precipitation of the M₂₃C₆ carbides will reduce the strength of the alloy and impair the high-temperature strength of the steel after long-term use. Therefore, the C content is limited to 0.08%-0.18%. Further, the optimal content of C should be limited to 0.10%-0.16%.

Silicon (Si): Si is an effective element for deoxidation of molten steel, and together with Cr, can improve the oxidation resistance of the steel. However, Si promotes the precipitation of Laves phases, which is not conducive to the toughness of the steel and has an adverse effect on creep strength. Therefore, the Si content is limited to 0.10%-0.40%. Further, the optimal content of Si should be limited to

Manganese (Mn): Mn can remove oxygen and sulfur in molten steel and improve the hardenability of steel. However, with the increase of the Mn content, the creep rupture strength decreases. Therefore, the Mn content is limited to 0.30%-0.70%. Further, the optimal content of Mn should be limited to 0.40%-0.60%.

Chromium (Cr): Cr in steel mainly functions to improve oxidation resistance and corrosion resistance, and it is an indispensable element as a constituent element of M₂₃C₆ carbides that improve high-temperature strength through precipitation strengthening. In order to achieve the above effects, the minimum Cr content in the heat-resistant cast steel of the present invention is 9.80%. However, if the Cr content exceeds 10.70%, it is easy to form δ-ferrite, which decreases the high-temperature strength and toughness. Therefore, the Cr content is limited to 9.80%-10.70%. Further, the optimal content of Cr should be limited to 10.00%-10.50%.

Molybdenum (Mo): The addition of Mo mainly increases the tempering stability of the steel and strengthens the secondary hardening effect. Moreover, the grain boundary segregation of Mo improves the bonding force of the grain boundaries, which increases the strength of the steel and reduces the loss of toughness. However, an excess of Mo leads to the formation of δ-ferrite and the precipitation of the intermetallic compound Laves phase, which obviously decreases the toughness. Therefore, the Mo content is limited to 0.45%-0.85%. Further, the optimal content of Mo should be limited to 0.55%-0.75%.

Tungsten (W): W is effective in inhibiting the coarsening of M₂₃C₆ carbides, and its effect exceeds that of Mo. The addition of W to replace part of Mo ensures that the Mo equivalent (Mo+½W) is about 1.5%, the creep strength is the best, and there is no excess of δ-ferrite formed. If the amount of W added exceeds 2%, the castings are prone to segregation. Therefore, the W content is limited to 1.60%-2.00%. Further, the optimal content of W should be limited to 1.65%-1.90%.

Cobalt (Co): Co, Mo and W are important elements that are distinctive from others in the present invention. Co can inhibit the formation of δ-ferrite after high-chromium ferritic steel is normalized or quenched at high temperature, fully exert the solid solution strengthening effect of Mo and W and increase the toughness of steel, which is critical for the heat-resistant cast steel of the present invention with high W content. The Co content is limited to 3.00%-3.50%. Further, the optimal content of Co should be limited to 3.10%-3.40%.

Vanadium (V) and Niobium (Nb): V and Nb are easily combined with C and N in the martensite to form MX carbonitrides. The MX carbonitrides that are precipitated finely and dispersedly greatly improve the strength of steel, and they are stable in long-term creep, which makes them the main strengthening phase. However, an excess of V and Nb will fix the carbon content excessively and reduce the precipitation of the M₂₃C₆ carbides, which leads to the decrease of the high-temperature strength. Moreover, Nb is easy to segregate in castings. Therefore, the V content is limited to 0.10-0.30%, and the Nb content is limited to 0.02%-0.08%. Further, the optimal content of V should be limited to 0.15%-0.25%, and the optimal content of Nb should be limited to 0.03%-0.07%.

Nickel (Ni): An appropriate amount of Ni can increase the hardenability of the steel, inhibit the formation of δ-ferrite and BN, and improve the strength and toughness at room temperature. However, an excessive addition is not conducive to the high-temperature creep properties of the steel. Therefore, the Ni content should be as low as possible. The Ni content is expected not to exceed 0.20%, and optimally not to exceed 0.10%.

Boron (B): B has the grain boundary strengthening effect, can form a solid solution in the M₂₃C₆ carbides, has the effect of inhibiting the coarsening of the M₂₃C₆ carbides, and can improve the high-temperature strength. The minimum content of B should be 0.001%. However, if the B content is0.010% or above, castability and weldability will be impaired. Therefore, the B content is limited to 0.001%-0.010%. Further, the optimal content of B should be limited to 0.002%-0.008%.

Nitrogen (N): N may precipitate VN (vanadium nitride) with V, which combines in the solid solution state with Mo and W to improve the high-temperature strength. The minimum content of N should be 0.01%. However, if the N content exceeds 0.04%, N may easily combine with B to precipitate BN, which impairs the creep properties of the steel. Therefore, the N content is limited to 0.010%-0.035%. Further, the optimal content of N should be limited to 0.015%-0.030%.

In the heat-resistant cast steel for castings provided by the present invention, the impurities include P and/or S and/or Al and/or Cu and/or Ti and/or Sn. S is a harmful impurity element in steel, which will reduce the thermoplasticity of the steel, affect hot workability and reduce corrosion resistance. The segregation of S at the grain boundaries will reduce the bonding force of the grain boundaries, resulting in a decrease of the high-temperature strength. P is also a harmful impurity element in steel, and if the P content is high, the steel will be brittle. Al may easily precipitate an AlN phase with N, which has adverse effects on the plasticity and toughness and the long-term creep properties of the steel. Sn may easily segregate at the grain boundaries, which significantly reduces the high-temperature strength of the alloy. As the impurity elements, P, S, Al, Cu, Ti and Sn have adverse effects on the mechanical properties of the heat-resistant cast steel and alloy, and their contents should be reduced as much as possible.

Table 1 shows a comparison of composition range medians of the heat-resistant cast steel for castings in the present invention and ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB specified in the industry standard JB/T 11018-2010.

TABLE 1 Comparison of chemical compositions (wt %) Heat-resistant cast steel CW2 of the Element present invention ZG12Cr10Mo1W1VNbN ZG13Cr9Mo2Co1NiVNbNB C 0.08-0.18 0.12 0.13 Si 0.10-0.40 0.30 0.25 Mn 0.30-0.70 1.00 0.90 P ≤0.030 ≤0.020 ≤0.020 S ≤0.020 ≤0.010 ≤0.010 Cr 9.80-10.70 9.70 9.25 Co 3.00-3.50 — 1.10 W 1.60-2.00 1.00 — Mo 0.45-0.85 1.00 1.60 V 0.10-0.30 0.20 0.20 Nb 0.02-0.08 0.06 0.06 N 0.010-0.035 0.050 0.0225 B 0.001-0.010 — 0.0115 Ni ≤0.20 0.70 0.35 Al ≤0.030 ≤0.020 ≤0.020

The present invention further discloses a preparation method of the heat-resistant cast steel described above, including the following steps:

Proportioning of raw materials is determined according to proportioning of components in the formula; the raw materials are melted, refined and poured into a mold; and then, quenching or normalizing is carried out, and finally tempering is carried out.

According to the technical solution of the preparation method described above, the quenching is carried out at 1080-1180° C. The tempering is carried out at a temperature of 700-780° C. The tempering is carried out one or more times.

The present invention further discloses use of the above-described heat-resistant cast steel in preparation of turbomachinery.

The present invention further discloses use of the above-described heat-resistant cast steel as a casting material in the field of steam turbines.

Compared with the existing casting material ZG12Cr10Mo1W1VNbN, Co and B are added to the composition of the present invention, the ratio of the solid solution strengthening elements Mo and W is adjusted, the contents of Mn, N and Ni are reduced, and the high-temperature creep strength is improved. Compared with the existing casting material ZG13Cr9Mo2Co1NiVNbNB, W is added to the composition of the present invention, the ratio of B and N is adjusted, the contents of Cr and Co are increased, the contents of Mn, Mo and Ni are reduced, and the high-temperature creep strength and oxidation resistance are improved. This will increase the operating temperature of the casting material, thereby improving the thermal efficiency of the generating unit and reducing the coal consumption and carbon dioxide emissions. The material designation of this novel heat-resistant cast steel is ZG12Cr10Co3W2MoVNbNB, referred to as CW2 for short.

The heat-resistant cast steel provided by the present invention may be used in preparation of turbomachinery, especially steam turbine castings. The obtained steam turbine castings have good high-temperature strength and oxidation resistance in a high temperature environment of 635° C. and below 635° C., and can satisfy the use requirements of steam turbines with a working temperature of 635° C. and below 635° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing results of an oxidation weight gain test of materials in embodiments of the present invention at 635° C.

DETAILED DESCRIPTION OF THE INVENTION

The implementations of the present invention are described below through specific embodiments. Those skilled in the art can easily understand the other advantages and effects of the present invention from the content disclosed in the specification.

Before further describing the specific implementations of the present invention, it should be understood that the protection scope of the present invention is not limited to the following specific implementation schemes. It should also be understood that the terms used in the embodiments of the present invention are intended to describe the specific implementation schemes, rather than to limit the protection scope of the present invention. The test methods that do not indicate specific conditions in the following embodiments usually follow the conventional conditions or the conditions recommended by the manufacturer.

When numerical ranges are given in the embodiments, it should be understood that unless otherwise indicated in the present invention, both endpoints of each numerical range and any numerical value between the two endpoints are all optional. Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the art. In addition to the specific methods, equipment and materials used in the embodiments, any method, equipment and material in the prior art similar or equivalent to those in the embodiments of the present invention may be used to implement the present invention according to the mastery of the prior art by those skilled in the art and the description of the present invention.

In the embodiments of the present application, proportioning of raw materials is determined according to proportioning of components in the formula; the raw materials are melted, refined and poured into a mold; and then, quenching or normalizing is carried out, and finally tempering is carried out.

Industrial pure iron is used as the raw material to serve as the source of Fe. Elemental carbon is used as the raw material to serve as the source of C. Industrial silicon is used as the raw material to serve as the source of Si. Electrolytic manganese is used as the raw material to serve as the source of Mn. Metallic chromium and chromium nitride are used as the raw materials to serve as the source of Cr. Electrolytic cobalt is used as the raw material to serve as the source of Co. Tungsten bars are used as the raw material to serve as the source of W. Metallic vanadium is used as the raw material to serve as the source of V. Niobium bars are used as the raw material to serve as the source of Nb. Chromium nitride is used as the raw material to serve as the source of N. Elemental boron is used as the raw material to serve as the source of B. Electrolytic nickel is used as the raw material to serve as the source of Ni.

Embodiment 1

According to the above theoretical calculation, certain amounts of raw materials were melted, refined, and poured into a mold to form a steam turbine cylinder, which was quenched at 1150° C. and tempered at 730° C.

Embodiment 2

According to the above theoretical calculation, certain amounts of raw materials were melted, refined, and poured into a mold to form a steam turbine valve casing, which was quenched at 1120° C. and tempered at 755° C.

Chemical composition analysis was carried out on the heat-resistant cast steel in Embodiment 1 and Embodiment 2. The analysis results are shown in Table 2 in wt %. Both satisfy the requirements of chemical composition indicators.

TABLE 2 Results of chemical composition analysis of heat-resistant cast steel for steam turbine castings in Embodiments 1 and 2 (wt %) Heat-resistant cast steel CW2 of the present invention Embodiment 1 Embodiment 2 C 0.08-0.18 0.15 0.11 Si 0.10-0.40 0.32 0.21 Mn 0.30-0.70 0.40 0.55 P ≤0.030 0.006 0.005 S ≤0.020 0.005 0.003 Cr 9.80-10.70 10.10 10.35 Co 3.00-3.50 3.15 3.35 Mo 0.45-0.85 0.58 0.69 W 1.60-2.00 1.85 1.70 V 0.10-0.30 0.15 0.21 Nb 0.02-0.08 0.04 0.06 N 0.010-0.035 0.015 0.025 B 0.001-0.010 0.0026 0.0060 Ni ≤0.20 0.10 0.05 Al ≤0.030 0.015 0.010

According to the industry standard JB/T 11018-2010, the mechanical property indicators of the existing casting materials ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB are listed in Table 3. The heat-resistant cast steel materials obtained in Embodiments 1 and 2 were subjected to the room temperature tensile test according to the standard GB/T 228.1, and subjected to the creep rupture strength test according to the standard GB/T 2039. Then, according to the extrapolation method specified in the standard GB/T 2039, the creep rupture strength limit R_(u 100,000 h/635° C.) at 635° C./100,000 h was deduced, and compared with the creep rupture strength of ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB at 635° C./100,000 h. The results are shown in Table 3. In Table 3, R_(p0.2) is the yield strength, and Rm is the tensile strength. As can be seen, the strengths (including R_(p0.2) yield strength and Rm tensile strength) obtained in Embodiment 1 and Embodiment 2 of the present invention satisfy the requirements of the indicators of ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB. In addition, the extrapolated value of the creep rupture strength of the material of the present invention is higher than 80 MPa, which is increased by 30% or above as compared with the extrapolated value of the creep rupture strength of the casting material ZG12Cr10Mo1W1VNbN, and by 20% or above as compared with the extrapolated value of the creep rupture strength of ZG13Cr9Mo2Co1NiVNbNB. As a result, the material of the present invention has an obvious strengthening effect, and can satisfy the use requirements of the steam turbine cylinder and valve casing with a working temperature of 635° C.

TABLE 3 Mechanical properties of heat-resistant cast steel for steam turbine castings in Embodiments 1 and 2 635^(o) C./ 100,000 h creep rupture strength R_(p0.2)/MPa R_(m)/MPa A/% Z/% KV₂/J MPa Embodiment 1 595 746 17 47 35 85 Embodiment 2 560 724 17.5 51 37 88 ZG12Cr10Mo1W1VNbN ≥520 680~850 ≥15 ≥40 ≥35 65 ZG13Cr9Mo2Co1NiVNbNB ≥500 630~750 ≥15 ≥40 ≥30 71

Embodiments 1 and 2, ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB were subjected to the oxidation weight gain test at 635° C. Samples of the four materials were placed in a flowing steam environment of 635° C. and 27 MPa for a maximum time of 2000 h. The weight gain of each sample was measured in this time period. The smaller the oxidation weight gain, the better the oxidation resistance of the material. The test results are shown in FIG. 1 . As can be seen from FIG. 1 , the oxidation resistance of Embodiments 1 and 2 is significantly better than that of ZG12Cr10Mo1W1VNbN and ZG13Cr9Mo2Co1NiVNbNB.

The above embodiments merely exemplarily illustrate the principles and effects of the present invention, but are not intended to limit the present invention. Any person skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those of ordinary skill in the art without departing from the spirit and technical ideas disclosed in the present invention should still be covered by the claims of the present invention. 

1. A heat-resistant cast steel, wherein based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel comprises the following elements in mass percentage: 0.08 wt %-0.18 wt % of C, 0.10 wt %-0.40 wt % of Si, 0.30 wt %-0.70 wt % of Mn, 9.80 wt %-10.70 wt % of Cr, 3.00 wt %-3.50 wt % of Co, 1.60 wt %-2.00 wt % of W, 0.45 wt %-0.85 wt % of Mo, 0.10 wt %-0.30 wt % of V, 0.02 wt %-0.08 wt % of Nb, 0.010 wt %-0.035 wt % of N, 0.001 wt %-0.010 wt % of B, ≤0.20 wt % of Ni and 79 wt %-85.5 wt % of Fe.
 2. The heat-resistant cast steel according to claim 1, wherein the heat-resistant cast steel further contains impurities, comprising one or more of Al, P, S, Cu, Ti and Sn.
 3. The heat-resistant cast steel according to claim 2, wherein based on the total mass of the heat-resistant cast steel, the corresponding mass percentages of Al, P, S, Cu, Ti and Sn are: ≤0.030 wt % of Al, ≤0.030 wt % of P, ≤0.020 wt % of S, ≤0.25 wt % of Cu, ≤0.030 wt % of Ti and ≤0.030 wt % of Sn.
 4. The heat-resistant cast steel according to claim 1, wherein based on the total mass of the heat-resistant cast steel, the heat-resistant cast steel comprises the following elements in mass percentage: 0.10 wt %-0.16 wt % of C, 0.20 wt %-0.30 wt % of Si, 0.40 wt %-0.60 wt % of Mn, 10.00 wt %-10.50 wt % of Cr, 3.10 wt %-3.40 wt % of Co, 1.65 wt %-1.90 wt % of W, 0.55 wt %-0.75 wt % of Mo, 0.15 wt %-0.25 wt % of V, 0.03 wt %-0.07 wt % of Nb, 0.015 wt %-0.030 wt % of N, 0.002 wt %-0.008 wt % of B, ≤0.10 wt % of Ni and 81 wt %-83.8 wt % of Fe.
 5. The heat-resistant cast steel according to claim 4, wherein the heat-resistant cast steel further contains impurities, comprising one or more of Al, P, S, Cu, Ti and Sn.
 6. The heat-resistant cast steel according to claim 5, wherein based on the total mass of the heat-resistant cast steel, the corresponding mass percentages of Al, P, S, Cu, Ti and Sn are: ≤0.020 wt % of Al, ≤0.020 wt % of P, ≤0.015 wt % of S, ≤0.15 wt % of Cu, ≤0.020 wt % of Ti and ≤0.020 wt % of Sn.
 7. A preparation method of the heat-resistant cast steel according to claim 1, wherein proportioning of raw materials is determined according to proportioning of components in the formula, and the raw materials are melted, refined and poured into a mold; and then, quenching or normalizing is carried out, and finally tempering is carried out.
 8. The preparation method according to claim 7, wherein the quenching or normalizing is carried out at a temperature of 1080-1180° C., and then the tempering is carried out at a temperature of 700-780° C., wherein the tempering is carried out one or more times.
 9. Use of the heat-resistant cast steel according claim 1 in preparation of turbomachinery or as a casting material in the field of steam turbines.
 10. (canceled) 